Conventionally, in a vessel such as a tanker and a shipping boat, an electric power amount consumed by various kinds of auxiliary machines, cargo-handling devices, illumination, air conditioners and other devices is enormous, and to supply electric power to the electric systems, the vessel has a diesel electricity generator composed of a combination of a diesel engine and an electricity generator for generating electric power when the diesel engine is driven (see Patent Document 1 for example). It is known that the diesel engine is one of engines having the highest energy efficiency among the internal combustion engines, and an amount of CO2 included in exhaust gas per unit output is small. Further, there is a merit that low quality fuel such as heavy oil can be used, and the diesel engine is also economically excellent.
In addition to CO2, large amounts of nitrogen oxide, sulfur oxide and particulate materials are also included in exhaust gas of the diesel engine. These materials mainly come from heavy oil which is fuel, and are harmful materials which inhibit environmental preservation. Especially nitrogen oxide (NOx, hereinafter) is harmful for human body and is strong acidic, and it is also considered that the NOx is a cause of acid rain. Therefore, a machine which drives a diesel electricity generator such as a vessel discharges an extremely large amount of NOx, and it is interpreted as giving a heavy burden to a global environment.
As post-processing means for largely purifying NOx, a selective catalyst reduction method (SCR method, hereinafter) using urea as reducing agent is generalized. In the SCR method, there is used NOx catalyst of a honeycomb structure made of material supporting active ingredient such as V and Cr on oxide carrier such as Ti. If urea water as reducing agent aqueous solution is sprayed to an upstream of NOx catalyst, the urea water is hydrolyzed by heat of exhaust gas and ammonia is produced, the ammonia exerts action on NOx as reducing agent, and NOx is dissolved into harmless nitrogen and water.